1. Field of the Invention
The present invention pertains to a semiconductor substrate handling blade which is used for handling substrates, typically thin wafers, during processing operations. Generally, the robot-operated substrate handling blade obtains the substrate from one location and transfers it to another within the processing system.
2. Brief Description of the Background Art
Semiconductor processing has been automated in recent years, to provide both efficiency in processing steps and to avoid contamination of the semiconductor substrate which might otherwise occur. As a part of this automation, semiconductor substrates, typically thin wafers, are frequently stored in cassettes to await further processing. In the most commonly used cassette designs, the wafers are horizontally oriented within the cassette with minimal spacing between each wafer. To place the wafers within the cassette and remove them without damage to or contamination of the wafers requires the use of specially designed robot-operated wafer handling equipment.
U.S. Pat. No. 4,620,738 to Schwartz et al., issued Nov. 4, 1986 describes a vacuum pick suitable for removing semiconductor wafers from and replacing wafers in a cassette holder. The vacuum pick includes a thin profile housing having a wafer support surface with a cavity therein, a resilient, flexible member covering a portion of the cavity to form an enclosure, and a rigid chuck mounted on the flexible member to permit movement of the chuck relative to the housing. Vacuum is applied to the enclosure so the wafer and the chuck are retracted against the housing and held firmly in place.
U.S. Pat. No. 4,705,951 to Layman et al., issued Nov. 10, 1987, discloses a wafer processing system including wafer handling arms incorporated into vacuum isolation valves. A loadlock with elevator and optical sensor is used to inventory and position a cassette of wafers. The wafers in the cassette can be randomly accessed.
U.S. Pat. No. 4,911,597 to Maydan et al., issued Mar. 27, 1990, describes a wafer processing system which includes an autoloader mounted within a load lock for providing batch, cassette-to-cassette automatic wafer transfer between the semiconductor processing chamber and cassette load and unload positions within the load lock. The system provides rapid, contamination-free loading and unloading of semiconductor wafers. Of particular interest herein, the processing system includes a shuttle blade and robotic wafer transfer system or robot. An indexer, the shuttle blade, and the robot work cooperatively to unload the wafers from containers such as cassettes onto wafer support hexodes and to offload the wafers from the hexode and return the wafers to the cassettes. The shuttle blade is a two-pronged blade which is mounted for generally horizontal pivotal movement to position the blade ends relative to particular loading and unloading stations. One blade is positioned to remove a wafer from the cassette while the other blade is positioned to off-load a processed wafer into the receiving cassette.
U.S. Pat. No. 5,387,067 to Howard Grunes, issued Feb. 7, 1995 describes a semiconductor cassette and transfer system for facilitating the direct and unloading of wafers from different sides of a cassette. Disclosed as a part of the transfer system is a robot blade which is fitted with a series of stepped edges which reduce the amount of wafer movement possible during loading/unloading operations, even if the wafer is misaligned with respect to the blade.
U.S. Pat. No. 5,483,138 to Shmooklet et al., issued Jan. 9, 1996, describes a system and method for automated positioning of a substrate in a process chamber. In particular, equipment is disclosed including a robot that can transfer a semiconductor substrate, such as a silicon wafer, from a cassette through a central transfer chamber and into one or more processing chambers about and connected to the transfer chamber in which the robot is located. An array of optical sensors is used to define the exact location of a semiconductor substrate relative to the processing chamber.
U.S. Pat. No. 5,556,147 to Somekh et. al., issued Sep. 17, 1996, discloses a wafer tray and a ceramic wafer carrying blade for semiconductor processing apparatus. The description of the apparatus itself and the process environments in which the apparatus is used serve as excellent background leading up to the present advance in the art. Somekh et. al. describes a ceramic wafer carrying blade with vacuum pick integral to the blade for transferring a wafer between storage cassette and transfer chamber. A removable wafer support tray is used in combination with the wafer carrying blade to move a wafer between a storage elevator and a processing chamber in an evacuated process environment.
Several of the above-described systems address the need for a wafer handling blade which reduces the amount of particulate contamination which can occur during handling of the wafers (due to rubbing between the surface of the wafer handling blade and the wafer); the need for an accurate means of determining the location of the wafer upon the handling blade; and the need for a means of compensating for misalignment which may occur between the blade and a wafer to be transported by the blade.
The majority of semiconductor substrate handling blades described above, and those commercially available and used within the industry are constructed from metal, and typically have a stainless steel main body. More recently developed handling blades have front and rear shoulders for holding the substrate in place (known as front and rear shoes). The front shoe is typically constructed from aluminum and the rear shoe from anodized aluminum. The Somekh et. al. reference is to an alumina ceramic handling blade, to provide structural strength under the high-heat operating conditions of the wafer processing reactors, even though the handling blade has an especially thin cross-sectional thickness.
The semiconductor handling blades have loading and unloading functions which produce particulate contamination from both the substrate surface and from the handling blade surface. The rubbing action which creates the particulates occurs when the substrate travels across the front shoe during uploading, when a vacuum chuck is used to pull the substrate surface against the surface of the handling blade, when the substrate travels across the surface of the back shoe as it settles into the space between the front and rear shoes, and when the substrate is off loaded from the handling blade.
One of the preferred wafer handling blade designs provides for wafer uploading or pick up in a manner so the front shoe of the handling blade sits in the center of the wafer, where the wafer is held in place by pulling a vacuum at an opening on the surface of the front shoe. The wearing between a silicon wafer surface and the front surfaces of the handling blade described above in this operation produces as many as 3,000 to 7,000 particles. This number might be even higher if the front shoe were anodized aluminum which would tend to gouge the wafer surface due to the abrasive nature of the anodized surface.
In addition to particulates created during vacuum chucking of a wafer, additional particulates are created during operations such as positioning the wafer between the front and back shoe so the wafer can be safely carried from a pre-processing storage area to a semiconductor processing chamber. First the wafer sits atop the front shoe of the blade, held in place by vacuum, then the vacuum is released and the blade slides forward and then backward beneath the wafer. The center of the wafer slides over the surface of the blade (toward the back of the blade) on the forward motion of the blade, and then the wafer drops into position between the front and rear shoe of the robot blade on the backward motion of the blade. The rear shoe is formed so that its leading edge is tapered at an angle. This permits the wafer to slide into a holding pocket between the front shoe and the rear shoe of the substrate handling blade, even when expansion differences between the handling blade and the semiconductor substrate would otherwise cause a misfit between a wafer and the holding pocket at particular process temperatures. The wafer typically sits at an angle of about 1.degree. between the front shoe and the rear shoe.
The sliding action of the wafer over the front and rear shoe surfaces causes rubbing between the wafer and the trailing edge of the aluminum front shoe, and rubbing over the surface of the anodized aluminum rear shoe. The particulates produced accumulate on the bottom of the semiconductor substrate and can fall from the bottom surface of one wafer to the top surface of another wafer while the wafers are stacked in storage cassettes; the particulates can fall upon processing chamber surfaces or migrate to the substrate surface when the substrate is exposed to particularly high temperatures (in the range of 650.degree. C.) during processing. The particulates travel by gravity, entrained in gas flow, and as a result of thermophoretic forces.
To further complicate matters, the wafer handling blade needs to be particularly thin, to negotiate its way between the narrowly spaced shelves in the storage cassette. The storage cassettes and the processing chambers are at elevated temperatures (frequently above 500.degree. C.), and the handling blade must be thin. This combination of requirements has resulted in the selection of metals having a high stiffness, with thermal stability at elevated temperatures as the material of construction for the wafer handling blade.
In addition to the optical sensing methods described above for locating a semiconductor substrate within a processing system, a cassette, or on a handling blade surface, an additional sensing capability for the presence of a substrate upon the handling blade surface was developed. The additional sensing capability used capacitance and a voltage measurement to sense the presence of a semiconductor substrate on a handling blade. A capacitive sensing circuit was either bonded to the upper surface of the robot blade, or portions of the sensing circuit were located within the blade, with an opening on the upper surface of the blade, through which the sensor was exposed.
Although the use of a capacitive circuit to sense the presence of the semiconductor substrate on the handling blade surface is beneficial, improved thermal stability is desired. The capacitance sensor bonded to the upper surface of a handling blade frequently becomes delaminated from the underlying blade over time, due to failure of the bonding adhesive and/or thermal expansion differences between the sensing device and the blade itself over the typical 400.degree. C.-650.degree. C. operational temperature range. Since the metal blade generally serves as a ground for the capacitance circuit; when the capacitance sensor becomes delaminated, this results in a failure in the capacitance circuit. When the capacitance circuit does not sense a wafer on the handling blade surface at the proper time, the system will not function as designed and generally must be shut down.
In addition, whether the capacitance sensing device was adhered to the handling blade surface or was exposed through an opening in the handling blade surface, the sensor was subjected to the process environment of the wafer it handled. As a result, the sensor often becomes corroded with time, producing contaminants within the wafer processing equipment; and, parts of the sensor are frequently degraded, outgassing materials into the wafer processing environment.
All of the above factors reduced the reliability of the sensor and caused contamination within the semiconductor processing environment.
There is clearly a need for an improved semiconductor substrate handling blade which reduces particulate contamination due to the wearing of both the wafer and the blade surface. In addition, it is highly desirable to have a more reliable capacitance-based sensor means, which does not contaminate the process environment, which does not delaminate from the handling blade, and which provides a more reliable, consistent signal range at higher temperatures.